Phased array antennas, both transmit and receive, typically consist of closely spaced individual antenna elements. The close spacing of these elements introduces cross coupling effects which dominate antenna performance characteristics. In addition, the antenna elements are designed for maximum power conversion efficiency between a radiation mode and a transmission line or circuit mode at the operating frequency of the antenna. This latter requirement consists of conjugate impedance matching of the impedance presented by the antenna terminal or port to the source impedance of a transmitter or the load impedance of a receiver.
The performance issues facing active phased array transmitters are radio frequency (RF) bandwidth, true time delay steering for wide bandwidth, electromagnetic interference (EMI) and beam steering control. Realizable active array transmitters providing this performance are limited in weight, size and generally costly.